1. Field of the Invention
A mobile telecommunications systems is disclosed, and in particular, to a message recovery method for a mobile device to increase call success rate.
2. Description of the Related Art
In a Universal Mobile Telecommunications System (UMTS) network, separation between the access stratum (AS) and the non-access stratum (NAS) is well defined by the Third Generation Partnership Project (3GPP) standard. FIG. 1 shows a message structure according to the 3GPP standard, presenting a layered structure as defined by the open system interconnection (OSI) model. For brevity, only relevant fields are illustrated in the message structure. A non-access stratum (NAS) message 110 is embedded in the body of the access stratum (AS) message 100, comprising certain header fields such as a core network (CN) domain identifier 112 and a protocol discriminator 114, and a message body that may further be embedded with an Internet Protocol (IP) packet 120 or other types of payloads. For example, the IP packet 120 is typically used to contain a Transmission Control Protocol (TCP) packet 130 embedded with the Hypertext Transport Protocol (HTTP) packet 140.
According to the UMTS standard, the NAS message 110 is passed between a User Equipment (UE) and the core network (not shown) through Radio Resource Control (RRC) initial/uplink/down direct transfers. The CN domain identifier 112 in the NAS message 110 is designated to provide specific information for CN domain identification. Based on the CN domain identifier 112, the UE is able to route the NAS message 110 to a proper message handler to handle the NAS message 110. Based on 3GPP specification, the domain contains at least two types, the packet switched (PS) domain, and the circuit switched (CS) domain. An NAS message 110 of the PS domain is typically used to present internet data packets, in which the protocol discriminator 114 typically contain values of GMM(0x08), SM(0x0a), GTTP(0x04) or SMS(0x09). Meanwhile, an NAS message 110 of the CS domain is generally phone call related data packets, and the protocol discriminator 114 are filled with one of the values MM(0x05), CC(0x03), SS(0x0b) and SMS(0x09).
Traffics for the PS domain and the CN domain are usually simultaneously processed if the UE supports both domains. For example, a mobile phone may be capable of surfing the internet concurrently while making a phone call. The interactions between the UE and the core network generally involve network entities such as Mobile Switching Centers (MSCs), Serving GPRS Support Nodes (SGSNs), Radio Network Controllers (RNCs) and Base Stations (BSs). Thus, sometimes the NAS message 110 may contain a CN domain identifier 112 with wrong domain values due to undeterminable reasons. The UE relies upon the CN domain identifier 112 to determine how to handle an NAS message 110 when receiving the NAS message 110. If the UE receives an NAS message 110 containing a CN domain identifier 112 of invalid value, the UE may ignore the NAS message 110. When such an error happens, a connection between the UE and the core network may fail or be disconnected, thereby degrading the service quality.
Therefore, a more robust message control mechanism is desirable.